Organic colloidal dispersion of iron particles, method for preparing same and use thereof as fuel additive for internal combustion engines

ABSTRACT

The colloidal dispersion of the invention is characterized in that it comprises an organic phase; particles of an iron compound in its amorphous form; and at least one amphiphilic agent. It is prepared by a process in which either an iron salt in the presence of an iron complexing agent or an iron complex is reacted with a base, maintaining the pH of the reaction medium at a value of at most 8 to obtain a precipitate, the iron complexing agent being selected from hydrosoluble carboxylic acids with a complexing constant K such that the pK is at least 3 and the iron complex being selected from the products of reacting iron salts with said acids; then the precipitate obtained or a suspension containing said precipitate is brought into contact with an organic phase in the presence of an amphiphilic agent to obtain the dispersion in an organic phase. The dispersion of the invention can be used as a combustion additive in liquid fuel or motor fuel.

The present invention relates to an organic colloidal dispersion of ironparticles, to a process for its preparation, and to its use as a fueladditive for internal combustion engines.

During combustion of gas oil in the diesel engine, carbonaceous productsare known to tend to form soot, which is known to be noxious both to theenvironment and to health. Techniques for reducing the emission of suchcarbonaceous particles, hereinafter termed “soot”, have long beeninvestigated.

One satisfactory solution consists of introducing catalysts into thesoot to allow frequent self-ignition of the soot collected on a filter.To this end, the soot has to have a sufficiently low self-ignitiontemperature that is frequently attained during normal operation of theengine.

It is known that dispersions of rare earth or iron compositions used asan additive can contribute to reducing the soot self-ignitiontemperature.

Such colloidal dispersions should have good dispersibility in the mediuminto which they are introduced, high stability over time and sufficientcatalytic activity at a relatively low concentration.

Known colloidal dispersions do not always satisfy all of those criteria.They may, for example, have good dispersibility but not sufficientstability, or good stability but a catalytic activity that requiresconcentrations that are too high to be of economic interest.

Further, they may have complex preparation processes. As an example,such dispersions are dispersions of particles in an organic phase andthey are generally obtained by transferring a starting dispersion in anaqueous phase into the final organic phase. Such a transfer can bedifficult to carry out.

The invention aims to provide a colloidal dispersion with improvedproperties the preparation of which is easier to carry out.

To this end, in a first aspect, the colloidal dispersion of theinvention is characterized in that it comprises:

-   -   an organic phase;    -   particles of an iron compound in its amorphous form;    -   at least one amphiphilic agent.

In accordance with a second implementation of the invention, theinvention also concerns a colloidal dispersion which is characterized inthat it comprises:

-   -   an organic phase;    -   particles of an iron compound in its amorphous form;    -   particles of a rare earth compound;    -   at least one amphiphilic agent.

The invention also concerns a process for preparing the dispersion inaccordance with said first implementation which is characterized in thatit comprises the following steps:

-   -   reacting either an iron salt in the presence of an iron        complexing agent or an iron complex with a base, maintaining the        pH of the reaction medium at a value of at most 8 to obtain a        precipitate, the iron complexing agent being selected from        hydrosoluble carboxylic acids with a complexing constant K such        that the pK is at least 3 and the iron complex being selected        from the products of reacting iron salts with said acids;    -   bringing the precipitate obtained or a suspension containing        said precipitate into contact with an organic phase in the        presence of an amphiphilic agent to obtain the dispersion in an        organic phase.

The dispersion of the invention has the advantage of being very stable.It also has high activity. The process for preparing the dispersion ofthe first implementation allows effective transfer of the aqueous phaseto the organic phase.

Further characteristics, details and advantages of the invention willbecome clearer from the following description, examples and figuresintended to illustrate it.

In the present description, the expression “colloidal dispersion”designates any system constituted by fine solid particles of an ironcompound or a rare earth compound, with colloidal dimensions, insuspension in a liquid phase, said particles possibly also containingresidual quantities of bound or adsorbed ions such as acetate orammonium ions, for example. It should be noted that in such adispersion, the iron or rare earth can be either completely in the formof colloids or simultaneously in the form of ions and in the form ofcolloids.

The dispersion of the first implementation of the invention will now bedescribed.

The dispersion of the invention is a dispersion in an organic phase.

This organic phase is selected as a function of the use of thedispersion.

The organic phase can be based on an apolar hydrocarbon.

Examples of the organic phase which can be cited are aliphatichydrocarbons such as hexane, heptane, octane or nonane, inertcycloaliphatic hydrocarbons such as cyclohexane, cyclopentane orcycloheptane, aromatic hydrocarbons such as benzene, toluene,ethylbenzene, xylenes or liquid naphthenes. Isopar or Solvesso(registered trade mark owned by EXXON) petroleum cuts, in particularSolvesso 100 which essentially contains a mixture of methylethyl- andtrimethyl-benzene, Solvesso 150 which comprises a mixture ofalkylbenzenes, in particular dimethylbenzene and tetramethylbenzene, andIsopar which essentially contains iso- and cycloparaffinic C-11 and C-12hydrocarbons, are also suitable.

It is also possible to use chlorinated hydrocarbons as the organic phasesuch as chloro- or dichloro-benzene or chlorotoluene. Ethers andaliphatic and cycloaliphatic ketones such as diisopropyl ether, dibutylether, methylisobutylketone, diisobutylketone or mesityl oxide can beenvisaged.

Clearly, the organic phase can be based on a mixture of two or morehydrocarbons of the type described above.

The particles of the dispersion of the invention are particles of aniron compound the composition of which essentially corresponds to aniron oxide and/or hydroxide and/or oxyhydroxide. The iron is generallyessentially present in oxidation state 3. The particles also contain acomplexing agent. The complexing agent corresponds to that which is usedin the process for preparing the dispersion either per se or in the formof an iron complex.

The particles of the dispersion of the invention are based on an ironcompound which is amorphous. This amorphous character can bedemonstrated by X ray analysis, as the X ray diagrams obtained do notshow any significant peaks.

In accordance with one characteristic of the invention, at least 85%,more particularly at least 90% and still more particularly at least 95%of the particles are primary particles. The term “primary particle”means a particle which is completely discrete and which is notaggregated with another or several other particles. This characteristiccan be demonstrated by examining the dispersion using TEM (highresolution transmission electron microscopy).

It is also possible to use the cryo-TEM technique to determine thedegree of aggregation of elementary particles. It allows transmissionelectron microscopic (TEM) examination of samples that are frozen intheir natural medium which is either water or organic diluents such asaromatic or aliphatic solvents, for example Solvesso and Isopar, orcertain alcohols such as ethanol.

Freezing is carried out on thin films about 50 nm to 100 nm inthickness, either in liquid ethane for aqueous samples or in liquidnitrogen for others.

The cryo-TEM preserves the degree of dispersion of the particles and isrepresentative of that present in the actual medium.

This characteristic of the particles of the dispersion contributes toits stability.

Further, the particles in the dispersion of the invention have a finegranulometry. They have a d₅₀ in the range 1 nm to 5 nm, moreparticularly in the range 3 nm to 4 nm.

The granulometry is determined by transmission electron microscopy (TEM)in conventional manner using a sample that has been dried on a carbonmembrane supported on a copper grid.

This technique for preparing the sample is preferred as it allows betteraccuracy in the particle size measurement. The zones selected for themeasurements are those which have a degree of dispersion similar to thatobserved in cryo-TEM.

The particles of the dispersion of the invention can have an isotropicmorphology, in particular with a ratio L (largest dimension)/l (smallestdimension) of at most 2.

The organic colloidal dispersion of the invention comprises at least oneamphiphilic agent with the organic phase.

This amphiphilic agent can be a carboxylic acid which generally contains10 to 50 carbon atoms, preferably 15 to 25 carbon atoms.

Said acid may be linear or branched. It can be selected from aryl,aliphatic or arylaliphatic acids, optionally carrying other functionsprovided that those functions are stable in the media in which thedispersions of the invention are to be used. Thus, for example, it ispossible to use aliphatic carboxylic acids, aliphatic sulphonic acids,aliphatic phoshonic acids, alkylarylsulphonic acids andalkylarylphosphonic acids, whether natural or synthetic. Clearly, it ispossible to use a mixture of acids.

Examples that can be cited include fatty acids of tall oil, soya oil,tallow, linseed oil, oleic acid, linoleic acid, stearic acid and theirisomers, pelargonic acid, capric acid, lauric acid, myristic acid,dodecylbenzenesulphonic acid, 2-ethylhexanoic acid, naphthenic acid,hexoic acid, toluenesulphonic acid, toluenephosphonic acid,laurylsulphonic acid, laurylphosphonic acid, palmitylsulphonic acid andpalmitylphosphonic acid.

Within the context of the present invention, the amphiphilic agent canalso be selected from polyoxyethylenated alkyl ether phosphates. Thismeans phosphates with formula:

or polyoxyethylenated dialkyl phosphates with formula:

in which formulae:

-   -   R¹, R² and R³, which may be identical or different, represent a        linear or branched alkyl radical, in particular containing 2 to        20 carbon atoms; a phenyl radical; an alkylaryl radical, more        particularly an alkylphenyl radical, in particular with an alkyl        chain containing 8 to 12 carbon atoms; or an arylalkyl radical,        more particularly a phenylaryl radical;    -   n represents the number of ethylene oxide units, which can be        from 0 to 12, for example;    -   M represents a hydrogen, sodium or potassium atom.

In particular, R₁ can be a hexyl, octyl, decyl, dodecyl, oleyl ornonylphenyl radical.

Examples of this type of amphiphilic compounds that can be cited arethose sold under the trade marks Lubrophos® and Rhodafac® by Rhodia andin particular the following products:

-   -   Rhodafac® RA polyoxyethylene (C8-C10)alkylether phosphates;    -   Rhodafac® RS710 or RS 410 polyoxyethylene tridecyl ether        phosphate;    -   Rhodafac® PA 35 polyoxyethylene oleodecyl ether phosphate;    -   Rhodafac® PA17 polyoxyethylene nonylphenyl ether phosphate;    -   Rhodafac® RE610 polyoxyethylene (branched)nonyl ether phosphate.

Finally, the amphiphilic agent can be a polyoxyethylenated alkyl ethercarboxylate with formula: R⁴—(OC₂H4)n—O—R⁵, in which R⁴ is a linear orbranched alkyl radical which can in particular contain 4 to 20 carbonatoms, n is a whole number which can, for example, be up to 12 and R⁵ isa carboxylic acid residue such as —CH₂COOH. Examples of this type ofamphiphilic compound that can be mentioned are those sold by KaoChemicals under the trade mark AKIPO®.

The dispersions of the invention have an iron compound concentrationwhich can be at least 8%, more particularly at least 15% and still moreparticularly at least 30%, this concentration being expressed as theequivalent weight of iron III oxide with respect to the total dispersionweight. This concentration can be up to 40%.

The dispersions of the invention have excellent stability. Nodecantation is observed after several months.

As indicated above, in a second implementation, the invention alsoconcerns a dispersion in an organic phase of particles of an ironcompound in its amorphous form and particles of a rare earth compound,as a mixture in an organic phase the dispersion further comprising anamphiphilic agent.

The above description concerning the first implementation of theinvention and concerning the nature of the organic phase and theamphiphilic agent is also relevant here.

Further, the rare earth in the rare earth compound can be selected fromcerium, lanthanum, yttrium, neodymium, gadolinium and praseodymium. Moreparticularly, cerium can be selected.

The particles of the rare earth compound can optionally have the samecharacteristics as those given above for the iron compound, inparticular as regards their dimensions or morphology. They could thushave a d₅₀ of the same value as that given above and also be primaryparticles, as indicated above.

The proportion of iron compound and rare earth compound can vary widely.However, the mole ratio of the iron compound/rare earth compound isgenerally in the range 0.5 to 1.5, and more particularly it can be equalto 1.

The rare earth compound can be a rare earth oxide and/or hydroxideand/or oxyhydroxide. Said compound can also be an organometalliccompound.

The process for preparing the dispersions of the invention in accordancewith the first implementation of the invention will now be described.

The first step of the process consists of reacting either an iron saltin the presence of a complexing agent or an iron complex with a base.This reaction is carried out in an aqueous medium.

Particular examples of the base can be hydroxide type products. Alkalior alkaline-earth hydroxides and ammonia can be cited. It is alsopossible to use secondary, tertiary or quaternary amines. However,amines and ammonia may be preferred provided that they reduce the riskof pollution by alkali or alkaline-earth cations. Urea can also bementioned.

Any water-soluble salt can be used as the iron salt. More particularly,ferric nitrate can be mentioned.

In accordance with a specific characteristic of the process of theinvention, the iron salt is reacted with the base in the presence of aniron complexing agent.

The iron complexing agents are selected from hydrosoluble carboxylicacids with a complexing constant K such that the pK is at least 3. Forthe reaction:

in which L designates the complexing agent, the constant K is defined asfollows:K=FeL _(x) ^(3-x) /[Fe ³⁺].[L⁻]^(x) and pK=log(1/k)

Acids having the above characteristics which can be mentioned arealiphatic carboxylic acids such as formic acid or acetic acid.Acid-alcohols or polyacid-alcohols are also suitable. Examples ofacid-alcohols that can be cited are glycolic acid and lactic acid.Polyacid-alcohols that can be mentioned are malic acid, tartaric acidand citric acid.

Other suitable acids that can be cited are amino acids such as lysine,alanine, serine, glycine, aspartic acid or arginine. It is also possibleto mention ethylene-diamine-tetraacetic acid or nitrilo-triacetic acidor N, N-diacetic glutamic acid with formula(HCOO⁻)CH₂CH₂—CH(COOH)N(CH₂COO—H)₂ or its sodium salt(NaCOO⁻)CH₂CH₂—CH(COONa)N(CH₂COO⁻Na)₂.

Other suitable complexing agents that can be used are polyacrylic acidsand their salts such as sodium polyacrylate, and more particularly thosethe mass average molecular mass of which is in the range 2000 to 5000.

Finally, it should be noted that a plurality of complexing agents can beused conjointly.

As indicated above, the reaction with the base can also be carried outwith an iron complex. In this case, the iron complex used is a productresulting from complexing iron with a complexing agent of the typedescribed above. This product can be obtained by reacting an iron saltwith said complexing agent.

The quantity of complexing agent used, expressed as the mole ratio ofcomplexing agent/iron, is preferably in the range 0.5 to 4, moreparticularly in the range 0.5 to 1.5 and still more particularly in therange 0.8 to 1.2.

The reaction between the iron salt and the base is carried out underconditions such that the pH of the reaction mixture which is formed isat most 8. More particularly, this pH can be at most 7.5 and it can inparticular be in the range 6.5 to 7.5.

The aqueous mixture and basic medium are brought into contact byintroducing a solution of the iron salt into a solution containing thebase. It is possible to carry out contact continuously, the pH conditionbeing satisfied by adjusting the respective flow rates of the solutionof iron salt and of the solution containing the base.

In a preferred implementation of the invention, it is possible tooperate under conditions such that during the reaction between the ironsalt and the base, the pH of the reaction medium formed is keptconstant. The term “pH is kept constant” means a pH variation of ±0.2 pHunits with respect to the fixed value. Such conditions can be achievedby adding an additional quantity of base to the reaction mixture formedduring the reaction between the iron salt and the base, for example whenintroducing the iron salt solution to the solution of the base.

The reaction is normally carried out at ambient temperature. Thisreaction can advantageously be carried out in an atmosphere of air ornitrogen or a nitrogen-air mixture.

At the end of the reaction, a precipitate is obtained. Optionally, theprecipitate can be matured by keeping it in the reaction medium for acertain period, for example several hours.

The precipitate can be separated from the reaction medium using anyknown means. The precipitate can be washed.

Preferably, the precipitate does not undergo a drying or freeze dryingstep or any operation of that type.

The precipitate can optionally be taken up in aqueous suspension.

However, it should be noted that it is entirely possible not to separatethe precipitate from the reaction medium in which it has been produced

To obtain a colloidal dispersion in an organic phase, either theseparated precipitate or the aqueous suspension obtained above afterseparating the precipitate from the reaction medium, or the precipitatein suspension in its reaction medium is brought into contact with theorganic phase in which the colloidal dispersion is to be produced. Thisorganic phase is of the type described above.

This contact is brought about in the presence of said amphiphilic agent.The quantity of this amphiphilic agent to be incorporated can be definedby the mole ratio r:$r = \frac{{number}\quad{of}\quad{moles}\quad{of}\quad{amphiphilic}\quad{agent}}{{number}\quad{of}\quad{moles}\quad{of}\quad{iron}\quad{elements}}$

This mole ratio can be in the range 0.2 to 1, preferably in the range0.4 to 0.8.

The quantity of organic phase to be incorporated is adjusted to obtain aconcentration of oxide as mentioned above.

At this stage, it may be advantageous to add to the organic phase apromoter agent the function of which is to accelerate transfer ofparticles of iron compound from the aqueous phase to the organic phase,if starting from a suspension of the precipitate, and to improve thestability of the organic colloidal dispersions obtained.

The promoter agent may be a compound with an alcohol function, moreparticularly linear or branched aliphatic alcohols containing 6 to 12carbon atoms. Specific examples that can be mentioned are2-ethylhexanol, decanol, dodecanol and mixtures thereof.

The proportion of said agent is not critical and can vary widely.However, a proportion in the range 2% to 15% by weight with respect tothe whole dispersion is generally suitable.

The order in which the different elements of the dispersion areintroduced is unimportant. The aqueous suspension, amphiphilic agent,organic phase and optional promoter agent may be mixed simultaneously.It is also possible to pre-mix the amphiphilic agent, organic phase andoptional promoter agent.

Contact between the aqueous suspension or the precipitate and theorganic phase can be made in a reactor which is in an atmosphere of air,nitrogen or an air-nitrogen mixture.

While contact between the aqueous suspension and the organic phase maybe made at ambient temperature, about 20° C., it is preferable tooperate at a temperature that is in the range from 60° C. to 150° C.,advantageously between 80° C. and 140° C.

In certain cases, because of the volatility of the organic phase, itsvapours may be condensed by cooling to a temperature below its boilingpoint.

The resulting reaction mixture (mixture of aqueous suspension,amphiphilic agent, organic phase and optional promoter agent) is stirredfor the whole heating period, which can vary.

When heating is stopped, two phases are observed: an organic phasecontaining the colloidal dispersion, and a residual aqueous phase.

The organic phase and aqueous phase are then separated usingconventional separation techniques: decantation, centrifuging.

In accordance with the present invention, organic colloidal dispersionsare obtained which have the characteristics mentioned above.

The dispersions of the second implementation of the invention can beobtained by mixing a first colloidal dispersion of particles of a rareearth compound in an organic phase with a second colloidal dispersion ofparticles of an iron compound, said second dispersion being inaccordance with the first implementation of the invention.

The first rare earth dispersion may be one as described in EP-A-0 206907, EP-A-0 671 205 or WO-A-00/49098, for example.

Preferably, dispersions the organic phases of which are identical aremixed.

The organic colloidal dispersions which have just been described can beused as a gas oil additive for internal combustion engines, moreparticularly as an additive for diesel engine gas oils.

They can also be used as combustion additives in liquid fuel or motorfuel for energy generators such as internal combustion engines (ignitionengines), domestic oil burners or reaction engines.

Finally, the invention concerns a motor fuel for internal combustionengines which contains a colloidal dispersion of the type describedabove or obtained by the process described above. This motor fuel isobtained by mixing it with the dispersion of the invention.

Examples will now be given.

EXAMPLE 1

Firstly, a solution of iron acetate was prepared.

412.2 g of 98% Fe(NO₃)₃,5H₂O was introduced into a beaker anddemineralized water was added to a volume of 2 litres. The solution was0.5 M in Fe. 650 ml of 10% ammonia was added dropwise, with stirring andat ambient temperature to produce a pH of 7.

It was centrifuged for 10 min at 4500 rpm. The mother liquor waseliminated. It was taken up in suspension in water to a total volume of2650 cm³. It was stirred for 10 min. It was centrifuged for 10 min at4500 rpm, then taken up into suspension in demineralized water to 2650cm³. It was stirred for 30 minutes. 206 ml of concentrated acetic acidwas then added. It was left overnight with stirring. The solution wasclear.

A solid was then precipitated in a continuous apparatus comprising:

-   -   a one litre reactor provided with a paddle agitator and an        initial stock constituted by 500 cm³ of demineralized water.        This reaction volume was kept constant by overflow;    -   two supply flasks containing the iron acetate solution described        above and a 10 M ammonium solution.

The iron acetate solution and the 10 M ammonia solution were added. Theflow rates of the two solutions were fixed so that the pH was keptconstant at 8.

The precipitate obtained was separated from the mother liquor bycentrifuging at 4500 rpm for 10 min. 95.5 g of recovered hydrate, 21.5%dry extract (i.e. 20.0 g equivalent of Fe₂O₃ or 0.25 mole of Fe), wasre-dispersed in a solution containing 42.7 g of isostearic acid and141.8 g of Isopar L. The suspension was introduced into a jacketedreactor provided with a thermostatted bath and a stirrer. The reactionassembly was heated to 90° C. for 5h30.

After cooling, it was transferred into a test tube. Demixing wasobserved and 50 cm³ of an aqueous phase and 220 cm³ of an organic phasewere recovered.

Completely discrete particles about 3 nm in diameter were observed bytransmission electron cryo-microscopy.

X ray analysis of the dispersion showed that the particles wereamorphous.

The dispersion underwent a heat. treatment which comprised constanttemperature stages at −20° C. and +80° C., cycling 6 times per day.After 6 months, no decantation was observed.

EXAMPLE 2

This example concerns an engine bench test employing the dispersion ofthe preceding example.

A Volkswagen 1.91 cylinder capacity turbo diesel engine with a manualgearbox, placed on a dynamometric rig was used. The exhaust line wasprovided with a 2.5 litre silicon carbide particle filter (IBIDEN 2000cpsi, 5.66×6.00). The temperature of the exhaust gas was measured at theparticle filter inlet using thermocouples. The pressure differentialbetween the inlet and outlet of the particle filter was also measured.

The organic dispersion obtained in the preceding example was added tothe fuel to provide a dose of 7 ppm of metal with respect to thesupplemented fuel.

The particle filter was charged with particles under the followingconditions:

-   -   rotation speed of engine: 2000 rpm;    -   torque 60 Nm;    -   inlet temperature of gas into filter: 250° C.;    -   charge duration: 8 hours

The soot trapped in the particle filter was burned under the followingconditions with an engine speed of 200 rpm, following a cycle comprising8 stages of 15 minutes each as described below: Stage Filter inlettemperature (° C.) Torque (Nm) 1 275 89 2 300 105 3 325 121 4 350 149 5375 231 6 400 244 7 425 254 8 450 263

The pressure drop created by the particle filter initially increasedbecause of the temperature increase then it reached a maximum beforedropping because of combustion of the carbonaceous materials accumulatedin the particle filter. The point (marked by its temperature) from whichthe pressure drop no longer increased was considered to represent theregeneration point of the particle filter by the additive.

During passage from stage 6 to stage 7, a reduction in the pressure dropwas observed, which corresponded to combustion of soot in the filter.The combustion onset temperature was in the range 400° C. 425° C. andmore precisely, it was 405° C. Soot combustion caused a reduction in thedrop at 425° C. of 6.49 mbar/min.

The results demonstrate a low regeneration temperature for a lowconcentration of additive in the fuel.

1-14. (Canceled)
 15. A colloidal dispersion, comprising: an organicphase; particles of an iron compound in its amorphous form; and at leastone amphiphilic agent.
 16. A colloidal dispersion, comprising: anorganic phase; particles of an iron compound in its amorphous form;particles of a rare earth compound; and at least one amphiphilic agent.17. The dispersion according to claim 15, wherein at least 85% of theiron compound particles are primary particles.
 18. The dispersionaccording to claim 16, wherein at least 85% of the iron compoundparticles are primary particles.
 19. The dispersion according to claim15, wherein the particles of the particles have a d₅₀ in the range 1 nmto 5 nm.
 20. The dispersion according to claim 19, wherein the particleshave a d₅₀ in the range 3 nm to 4 nm.
 21. The dispersion according toclaim 16, wherein the particles of the particles have a d₅₀ in the range1 nm to 5 nm.
 22. The dispersion according to claim 21, wherein theparticles have a d₅₀ in the range 3 nm to 4 nm.
 23. The dispersionaccording to claim 15, wherein the organic phase is based on an apolarhydrocarbon.
 24. The dispersion according to claim 16, wherein theorganic phase is based on an apolar hydrocarbon.
 25. The dispersionaccording to claim 15, wherein the amphiphilic agent is a carboxylicacid containing 10 to 50 carbon atoms.
 26. The dispersion according toclaim 16, wherein the amphiphilic agent is a carboxylic acid containing10 to 50 carbon atoms.
 27. The dispersion according to claim 16, whereinthe rare earth is cerium, lanthanum, yttrium, neodymium, gadolinium orpraseodymium.
 28. A process for preparing a dispersion as defined inclaim 15, comprising the following steps of: a) reacting in a reactionmedium either an iron salt in the presence of an iron complexing agentor an iron complex with a base, maintaining the pH of the reactionmedium at a value of at most 8 to obtain a precipitate, the ironcomplexing agent being selected from hydrosoluble carboxylic acids witha complexing constant K such that the pK is at least 3 and the ironcomplex being selected from the products of reacting iron salts withsaid acids; and b) adding the precipitate obtained in step a) or asuspension containing said precipitate to an organic phase in thepresence of an amphiphilic agent; and c) obtaining from step b) saiddispersion in an organic phase.
 29. The process according to claim 28,wherein said carboxylic acid in step a) is a carboxylic acid,acid-alcohol, polyacid-alcohol, amino acid or polyacrylic acids.
 30. Theprocess according to claim 29, wherein the aliphatic carboxylic acid isformic acid, acetic acid, tartaric acid or citric acid.
 31. The processaccording to claim 28, wherein the pH of the reaction medium is kept ata value of at most 7.5.
 32. The process according to claim 31, whereinthe pH is in the range 6.5 to 7.5.
 33. The process for preparing adispersion as defined in claim 16, comprising the steps of: a) preparinga mixture of a colloidal dispersion of particles of a rare earthcompound in an organic phase; b) mixing said mixture with a colloidaldispersion of particles of an iron compound; and c) obtaining saiddispersion from step b).
 34. A motor fuel for internal combustionengines, comprising a colloidal dispersion as defined in claim
 15. 35. Amotor fuel for internal combustion engines, comprising a colloidaldispersion as defined in claim 16.